Added "filter" level for var filter to match other filters (#267)

docs/REST_API.md

@@ -512,6 +512,7 @@ If differential expression is not supported by the server, must return an HTTP 5
       {
         "mode": "varFilter",
         "varFilter": {
+          "filter": {
             "var": {
               "annotation_value": [
                 { "name": "gene", "values": [ "FOXP2", "ACTN1" ] }
@@ -519,6 +520,7 @@ If differential expression is not supported by the server, must return an HTTP 5
             }
           }
         }
+      }
   ```
 
 - set1 - Complex filter - defines observation "set1"